1. Field of the Invention
The invention is directed to an optical waveguide amplifier for use in telecommunication systems and more particularly, a waveguide optical amplifier that reduces nonlinear signal impairments within the optical amplifier.
2. Technical Background
The continuous growth of bandwidth requirements in optical-based communication systems has resulted in a large demand for systems able to operate within several optical wavelength ranges including the S-band optical range, the C-band optical range and the L-band optical range. The S-band is defined as the wavelengths between about 1465 nm and about 1525 nm, which lies below the C-band wavelength range which extends between about 1525 nm and about 1560 nm, which in turn lies just below the L-band wavelength range which extends between about 1560 nm and about 1610 nm. In order to create a viable operating bandwidth, a large bandwidth must be obtained within each of the operating wavelength ranges. Currently, most telecommunications systems utilize the C-band and L-band ranges.
The growth of bandwidth requirements has also resulted in a demand for higher data rates in optical communication systems resulting in higher optical powers propagating within the optical fibers associated with the communication systems. Specifically, optical amplifier technology and wavelength division multiplexing techniques are typically required in numerous telecommunication systems, such as those systems that require high-power transmissions for long distances, as well as dense wavelength-division multiplexing technology (DWDM) used in metropolitan area networks.
With respect to high-power transmissions for long distances, the definition of high-power and long distances is meaningful only in the context of a particular telecommunication system wherein a bit rate, a bit error rate, a multiplexing scheme, and optical amplifiers are specified. There are additional factors, known to those skilled in the art, which have impacted upon the meaning of high power and long distance, however, for most purposes, high power is a total optical power greater than about 10 mW or single channel power of greater than 1 mW. In some applications, optical amplifiers with single channel power levels of 1 mW or less are also sensitive to non-linear effects. A long distance is one in which the distance between electronic regenerators can be in excess of 100 km. The regenerators are to be distinguished from repeaters which make use of optical amplifiers. Repeater spacing, especially in high data density systems, can be less than half the regenerator spacing.
Several problems are associated with utilizing higher optical powers to propagate signals in optical fibers, including less tolerance for nonlinear-optical impairments associated with such propagations. In addition, the expansion of wave division multiplexing technology into the L-band frequency range has highlighted the problems associated with these nonlinear effects within erbium-doped fiber amplifiers (EDFAs). These nonlinear-optical impairments may become the dominant signal degradation factor of the EDFAs.
Of the nonlinear-optical impairments associated with the amplification of high-power optical signals within EDFAs, the most notable nonlinear-optical impairments include four-wave mixing, cross-phase modulation, and self-phase modulation. Four-wave mixing occurs when two or more frequencies of light propagate through an optical waveguide fiber together, and phase-matching occurs between the light signals. Light is then generated at new frequencies using optical power from the original signals. Cross-phase modulation occurs between two waves having different frequencies and similar polarizations or the same frequencies but different polarizations. Self-phase modulation is a temporal analog of self-focusing within an optical waveguide fiber, and results in spectral broadening of optical pulses.
The problem of optically amplifying optical signals within the L-band range, while minimizing noise generated during amplification and while simultaneously minimizing the amount of nonlinear impairments introduced into the signal, is critical to amplifier design.
A solution is needed, therefore, which allows for the amplification of an optical signal while simultaneously minimizing nonlinear-optical impairments and optimizing the optical signal-to-noise ratio developed within the associated optical amplifier.
One aspect of the present invention is to provide a method for amplifying an optical signal within an optical waveguide amplifier, including providing at least one optical waveguide amplifier having an input for receiving an optical source signal therein, and an output, wherein a forward pumping direction extends from the input to the output and rearward pumping direction extends from the output to the input. The method also includes providing at least one excitation light source in optical communication with the optical waveguide amplifier and capable of generating at least one excitation light. The method further includes amplifying the source signal by pumping a first excitation light from the excitation light source in the rearward pumping direction, and amplifying the source signal by pumping a second excitation light from the excitation light source in the forward direction simultaneous with pumping the first excitation light source.
According to one embodiment of the present invention an optical waveguide amplifier comprises at least one gain producing optical waveguide having an input for receiving an optical source signal therein and an output. The optical waveguide amplifier has a forward pumping direction that extends from the input to the output and a rearward pumping direction that extends from the output to the input. The optical amplifier further includes at least one excitation light source in optical communication with the optical waveguide, the excitation light source generating at least one excitation light. The source signal is amplified by pumping (i) a first excitation light from the excitation light source into the optical waveguide amplifier in the rearward pumping direction, and (ii) a second excitation light from the excitation light source into the optical waveguide amplifier in the forward direction. Finally, the optical amplifier is configured to satisfy the following equations: |xcex94OSNR| less than 0.4 dB and FWMX xe2x89xa6xe2x88x9225 dB, where OSNR stands for signal to noise ratio and, FWMX is four wave mixing cross talk.
According to an embodiment of the present invention an optical fiber communications system comprises: a transmitter; a receiver; at least one optical waveguide amplifier having (i) an input in optical communication with the optical waveguide amplifier for receiving the source signal and (ii) an output in optical communication with the receiver for delivering the source signal to the receiver. The optical waveguide amplifier has a forward pumping direction that extends from the input to the output and a rearward pumping direction that extends from the output to the input. The optical communication system further includes at least one excitation light source in optical communication with the optical waveguide amplifier, the excitation light source generating at least one excitation light. The source signal is amplified by pumping (i) a first excitation light from the excitation light source into the optical waveguide amplifier in the rearward pumping direction, and (ii) a second excitation light from the excitation light source into the optical waveguide amplifier in the forward direction, wherein   1.5   less than             P      r              P      f         less than   12
and FWMX less than xe2x88x9230 dB, where Pr is rearwardly propagating pump power, Pf is forward propagating pump power, and FWMX is four wave mixing cross talk.
In addition, the details of embodiments of an apparatus capable of utilizing the above-referenced method and telecommunication systems capable of utilizing the same are disclosed and described herein.
Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings.